Pigments are widely used in many industries to impart basically a pleasing, esthetic color and appearance to various media in which they are mixed, such as paints, varnishes, lacquers, organic resins, glazes, ceramic ware such as porcelain enamels, glass, and the like. As a general rule, inorganic pigments have a greater scope of application as compared to organic pigments. Because of the more severe and drastic conditions of use in glazes and ceramic applications, particularly with reference to temperature of preparation, inorganic pigments must be used for such applications. However, inorganic pigments may also be added to organic media, such as organic resinous polymeric materials, especially when such materials are used at relatively high temperatures such as in extruding or molding.
Important inorganic red pigments include those of the cadmium sulfide-cadmium selenide series. Two additionally important inorganic yellow pigments are based on cadmium sulfide and lead antimonate (Naples Yellow). As awareness of the need for environmental protection grows, industrial use of toxic and semi-toxic chemicals has come under greater scrutiny and criticism. The use of such metals as cadmium, lead and antimony, previously extensively employed in preparing pigments, has accordingly become less popular. Stronger governmental regulation and control of their use may arise. It is, therefore, important to develop substitute pigments.
Several techniques have been previously suggested for varying the color of a pigment. One technique comprises changing the degree of fineness of the pigment grind. For example, an orange pigment comprising 99% zinc oxide and 1% manganese oxide may be altered toward yellow by increasing the fineness of the grind.
Another technique is to combine two pigments of different colors to obtain a color existing somewhere between the colors of the two pigments. For instance, blue-green pigments may be prepared in this manner by mixing a green pigment and a blue-violet pigment in proper proportions. By careful selection, a mechanical mixture of the two pigments can result in a final color mixture which has acceptable color density. However, in many instances, mixtures of different pigments result only in muddy colors.
Still another technique is to add to a basic pigment a color shifting agent in which the color absorption characteristics of the pigment is changed or shifted slightly from that which would otherwise prevail. The present invention relates to the color shifting technique for modifying the color of a pigment.
U.S. Pat. No. 4,086,123 to Hummel and Sombuthawee discloses pigments comprising zinc sulfide as a host containing in solid solution a metal sulfide dopant. The dopant may comprise one or more certain metal sulfides, but normally is present as a unitary sulfide, such as lithium sulfide, or preferably as a multiple sulfide, such as lithium indium sulfide and copper indium sulfide.
U.S. patent application Ser. No. 772,233, filed Feb. 25, 1977, now abaondoned, in the names of Hummel and Bonsall and assigned to the same assignee to which this application is assigned, discloses pigments containing zinc sulfide as a host containing in solid solution a dopant to alter the color of the zinc sulfide. The dopant may comprise one or more certain metal sulfides, but normally is present as a unitary sulfide, such as indium sulfide, or preferably as a binary sulfide, such as indium sulfide and manganese sulfide.
U.S. Pat. No. 2,579,020 to Smith discloses pigments of zinc oxide in combination with a coloring metal oxide and having as a color shifting agent magnesium oxide or cadmium oxide.